This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. Human centrin 2 (20 kDa) and calbindin D28K (30 kDa) are members of the family of Ca2+-binding proteins. We use PLIMSTEX, the study of Protein Ligand Interactions by Mass Spectrometry, Titration, and H/D Exchange, to explore the calcium binding properties of these proteins. This strategy can show the conformational change upon binding calcium, the affinity constants, and also the binding stoichiometry.